1. Field of the Invention
This invention relates to heat reflecting films. More particularly, it relates to composite films comprising a series of dielectric and metal layers so as to create an infra-red reflecting interference filter and to the use of such films in window glazing materials.
2. Description of the Prior Art
In the 1890's, Fabry and Perot developed an interferometer consisting of a pair of parallel-sided half-silvered mirrors separated by a nonabsorbing layer. This device had the property of preferentially passing energy of certain wavelengths and reflecting energy of other wavelengths. An embodiment of this principle known as the Fabry-Perot sandwich consists of two more or less transparent metal layers separated by a dielectric spacer layer. (See, for example, Knittl, Zdenek; OPTICS OF THIN FILMS, John Wiley & Sons, Ltd., London, 1976, at page 284.) Other filter products known as "induced transmission filters" have been constructed of metal-dieletric sandwiches for use in window glazing structures. One such structure is described in U.S. Pat. No. 4,337,990 of Fan (July 6, 1982) as consisting of a transparent substrate, overlayered with a phase matching layer, a single metallic silver layer and an outer antireflection layer, with the three overlayers constituting a transparent heat reflector. While generally effective, products of this general structure suffer from the disadvantage that to achieve high levels of heat reflection they must have relatively thick metal layers such as 15 to 25 nm in thickness which tend to have low transmittances of visible radiation, as well.
Another system which used a Fabry-Perot approach to achieve heat reflection while transmitting visible radiation is shown in U.S. Pat. No. 3,682,528 of Apfel and Gelber Aug. 8, 1972). In this system, thinner layers of metal are employed but it is taught that to obtain such layers of an optically suitable metal, in particular silver, it is necessary to first lay down a thin "nucleation" precoat layer of nickel by vacuum deposition and then apply the silver to it, again by vacuum deposition methods. It is further taught that the deposited silver must then receive a thin postcoat layer of vapor-deposited nickel if another layer is to be applied over it. These extra coatings with nickel are time consuming and economically unattractive. This patent also discloses a filter having two silver layers but shows that each silver layer must be accompanied by one or two nickel layers and suggests only durability advantages to this more involved structure. The substrate upon which this multilayer heat reflecting film was constructed most commonly was glass which, being rigid and heavy in weight, added to the difficulties of fabrication.